Field of the Invention
The field of the invention is joining practice, and in particular welding practice.
The invention relates to a method of welding two work pieces that define a connecting zone between them. The invention also relates to an apparatus for welding two work pieces that define a connecting zone between them, having a friction pin that can be advanced into the connecting zone.
To join two work pieces, for example work pieces made of aluminum or an aluminum alloy, it is known, for example from European Patent EP 0 615 480 B1, to bring about the plasticizing or melting of the parent material by a heat input produced by friction, this plasticizing or melting being necessary for welding at the abutting, joining or connecting zone of the work pieces. Provided for this purpose is a friction welding head that contains a rapidly and cyclically moving, for example rotating, friction pin. The friction pin is made of a material that is harder than and has a higher melting point than the material of the work pieces to be joined. When two plate-shaped work pieces are being joined, the friction pin is placed laterally against the work pieces in the region of the connecting zone of the work pieces. The friction pin, on account of the plasticizing of the material caused by the rapid and cyclical movement, then penetrates into opposite regions of the work pieces, while simultaneously being moved forward along the connecting zone, and thus brings about the welding of the two work pieces during the cooling of the plasticized zone. The method is also designated as friction stir welding.
Developments and modifications of friction stir welding are described in International Patent Disclosures WO 95/26254 and WO 96/38256.
Work pieces in which the weld does not extend up to a marginal region of the work pieces often have to be welded to one another, so that the friction pin-after being inserted laterally into the work pieces-cannot leave the work pieces again laterally at another point. For example, plate-shaped work pieces must be welded only in sections along an abutting edge, in which case at least one of the sections does not extend up to the margin of the plates. In this use, there is the problem that an undesirable hole is left at the end of a weld on account of the material displacement of the friction pin when withdrawing or lifting it.
This problem also exists if a weld having a shape closed upon itself is to be produced. In particular, this problem exists when (tightly) welding two tubes, since in this case the friction pin, after a complete orbit along the tube circumference, returns to the starting point again.
In German Patent DE 196 16 285 C1, it is proposed to withdraw the friction pin slowly from the tube wall in the circumferential direction while maintaining the orbiting movement, so that a wedge-shaped run out is obtained. However, this does not completely prevent the hole produced during the radial lifting. In the region of the wedge-shaped run out, the welded tubes often have an undesirable reduction in wall thickness.
To close an undesirable bore or an undesirable hole in a work piece, it is known, for example from Published, British Patent Application GB 22 33 923 A, to insert a rapidly rotating plug into the hole while producing friction heat and to tightly close the hole with the plug. Such a procedure in connection with the welding of many tubes would be expensive, since such a friction plug would have to be supplied after every complete orbit of the friction pin, for which purpose a separate device or at least a tool change would possibly be necessary.
It is accordingly an object of the invention to provide a method and an apparatus for welding two work pieces which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, in which a hole at the finishing point of the weld, at which lifting of the friction pin takes place, is reliably avoided with little technical outlay.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of welding two work pieces that define a connecting zone between them. The method includes the steps of feeding a friction pin at a starting point into the connecting zone and into opposite regions of the work pieces on both sides of the connecting zone; moving the friction pin in a welding direction along the connecting zone resulting in the work pieces being welded; leading the friction pin out of the connecting zone after welding the work pieces; and feeding the friction pin into a sacrificial element while lifting the friction pin.
In this connection, sacrificial element refers to any element that is not required for the function of the two work pieces, for example two tubes. The method therefore achieves the advantage that the hole remaining when the friction pin is lifted does not appear in a functionally important region of the work pieces but in the sacrificial element. For example, the sacrificial element projects above the outer surface of two tubular work pieces, so that, when the hole is produced in the sacrificial element, the wall thickness of the tubes does not drop below the desired value at any point.
The sacrificial element may have any desired shape, for example the shape of a nose, a projecting length or an extension.
The friction pin works in particular according to the friction stir principle, which is accompanied by plasticizing of the opposite regions of the work pieces on both sides of the connecting zone.
In connection with the invention, lifting of the friction pin refers to any withdrawal of the friction pin from one of the work pieces or from the work pieces with at least one motion component in a direction perpendicular to the direction of the connecting zone, that is to the welding direction.
According to a preferred refinement, the sacrificial element is integrally formed on one of the work pieces as an integral part of the work piece in such a way as to form a sacrificial region. For example, the sacrificial region and the work piece are formed or produced from a single piece. In particular, the sacrificial region projects above a surface of one of the work pieces.
According to a preferred refinement, the sacrificial element is configured as a separate sacrificial body that is attached so as to be adjacent to at least one of the work pieces. Such a sacrificial body is simple to produce and requires no additional outlay when producing the work pieces.
For example, the sacrificial body is adapted at its underside to the contour of the work piece, so that as far as possible seating without a gap or free of gaps is obtained. The sacrificial bodyxe2x80x94and also the sacrificial regionxe2x80x94preferably also extends in a direction perpendicular to the welding direction.
In an especially advantageous manner, the sacrificial body or the sacrificial region is made of the same material as the work pieces. This ensures that the hole remaining in the work pieces without the use of a sacrificial element is avoided or closed in an especially reliable manner by material from the sacrificial element.
The method is preferably used in such work pieces in which the connecting zone has a shape closed upon itself, in particular a ring shape. Such work pieces are tubes for example. In this case, the weld extends from the starting point up to a finishing point, the finishing point being identical to the starting point. For example, in the case of tubes, welding is carried out along a complete circumferential line (360xc2x0).
In a connecting zone having a shape closed upon itself, the friction pin, after completely covering the connecting zone, is preferably moved beyond the starting point. When two tubes are being welded, the friction pin performs an orbiting movement of more than 360xc2x0, that is to say beyond the closing point.
In this case, for example, a section of the connecting zone that is covered repeatedly by the friction pin is obtained. By such an overlap, which is set at a machine for example, the error tolerance of the method is advantageously increased.
In particular, the sacrificial elementxe2x80x94as viewed in the welding directionxe2x80x94is attached behind the starting point, which in particular is identical to the finishing point.
According to an especially preferred refinement, in a connecting zone having a shape closed upon itself, the sacrificial element is attached at the starting point, or at a point which has already been passed by the friction pin, after the friction pin has already been moved in the welding direction. The sacrificial body is attached, for example, at an instant at which there is sufficient space for attaching it above the starting point or above a point that has already been passed by the friction pin.
During such a procedure, the friction pin, after a complete orbit, runs virtually automatically into the sacrificial body without a change in its (azimuthal) direction of movement being necessary or without an additional movement being necessary.
Alternatively, the sacrificial elementxe2x80x94as viewed in the welding directionxe2x80x94is attached laterally next to the starting point, or next to a point which has already been passed by the friction pin, after the friction pin has already been moved in the welding direction. In this case, after a complete orbit of the friction pin, it may be necessary to end or change the azimuthal movement of the friction pin in such a way that the friction pin is moved into the sacrificial body laterally next to the starting point, or next to the point which has already been passed by the friction pin, after a forward movement in one of the work pieces.
According to another especially preferred refinement, the sacrificial element is attached so as to be directly adjacent to the connecting zone. This achieves the advantage that, after complete welding along the connecting zone, the friction pin can be fed into the sacrificial element without delay. In this case, a separate forward movement in one of the work pieces may not be necessary, and the lifting of the friction pin when it is being fed into the sacrificial element can take place when it is being led out of the connecting zone.
In order to keep delays to a minimum, the sacrificial element is attached at least at a distance from the connecting zone of less than the diameter of the friction pin.
According to another preferred refinement, the sacrificial elementxe2x80x94as viewed in the welding directionxe2x80x94is attached laterally next to the connecting zone. This refinement is especially advantageous if the sacrificial element, as sacrificial region, is an integral part of one of the work pieces. The sacrificial region can then be integrally formed in an especially simple manner at a distance from the connecting zone without it having an adverse effect on positioning of the work pieces to be welded along the connecting zone and/or on the movement of the friction pin.
In a preferred development of the method, the sacrificial element is removed after the welding of the work pieces. The removal of the sacrificial element, once the friction pin is led out of it, may be effected, for example, by milling or cutting.
Another preferred development provides for a feed movement of the friction pin to be maintained during the lifting of the friction pin. The superimposition of the two movements results in an especially neat runout point.
The height of the sacrificial element relative to that surface of the work piece or of the work pieces which surrounds it is preferably greater than the weld depth produced. Since the weld depth produced is related to the height of the friction pin (friction surface plunging in), this ensures that the entire friction pin plunging in can be accommodated by the sacrificial element.
In this case, in particular, the expression xe2x80x9cheightxe2x80x9d refers to the height perpendicular to the surface of the work pieces. For example, the sacrificial element projects by this height above the connecting zone perpendicularly to the welding direction.
According to another exceptionally preferred refinement, the height of the sacrificial element relative to that surface of the work piece or of the work pieces which surrounds it is spatially variable. As a result, in particular in combination with an overall movement of the friction pin superimposed from the lifting and the feed movement, a simple and neater transition from the connecting zone into the sacrificial element is achieved. In particular, the height increases (continuously) in a direction that is parallel to the direction of the feed movement of the friction pin.
According to another preferred refinement, the friction pin is therefore fed into the sacrificial element in the direction of increasing height of the sacrificial element.
For example, the sacrificial element is wedge-shaped. When two tubes are being welded, the shape of the wedge is in particular such that its top side or outer side runs at least partly tangentially to the outer circumference of the tubes.
The friction pin is preferably lifted or withdrawn in a controlled manner in such a way as to follow the contour of the sacrificial element.
The object which relates to the apparatus is achieved with respect to the apparatus of the type mentioned at the beginning in that there is a control device with which the friction pin can be withdrawn from the work piece or the work pieces in a controlled manner and with which a feed movement of the friction pin can be performed at the same time. The apparatus for welding two work pieces includes the friction pin which can be advanced into the connecting zone and can be fed into a sacrificial element, and a control device with which the friction pin can be withdrawn from the work pieces in a controlled manner and in such a way as to follow a contour of the sacrificial element and with which a feed movement of the friction pin can be performed at the same time.
The apparatus is suitable in particular for carrying out the method according to the invention. Especially advantageous in this case is the use of the apparatus if a sacrificial element having a spatially variable height is used.
The friction pin can preferably be withdrawn in an automated manner as a function of the path covered in the feed direction.
An advantageous development of the invention has a scanning body scanning the path and controlling the withdrawal movement or the lifting of the friction pin.
The scanning body and the friction pin are preferably connected via a control cylinder.
The friction pin can preferably be withdrawn in such a way as to follow a cam firmly superimposed on the control device. The cam may be realized mechanically or electronically.
According to an especially preferred refinement, the cam represents the contour of a sacrificial element.
The invention also relates to a sacrificial body for carrying out the method. The sacrificial body is in particular a separate body that can be placed against at least one of the work pieces. Possible refinements and advantages of the sacrificial body can be gathered from the description of the method according to the invention.
Furthermore, the invention also relates to a work piece having a sacrificial region for carrying out the method according to the invention. In particular, the sacrificial body is integrally formed on one of the work pieces as an integral part of this work piece.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and an apparatus for welding two work pieces, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.